The invention relates to a sealing element for a face seal assembly, and particularly to a sealing element which enables the construction of a face seal assembly having specifically advantageous, but not exclusive, utility for sealing aggressive media.
For sealing aggressive media use has hitherto mainly been made of double action, fluid buffered face seal assemblies, in which only the outer portions thereof subjected directly to the aggressive media are formed of chemically resistant materials. However, double action face seal assemblies are built in a comparatively complex manner and are consequently expensive. In addition, they require a high level of maintenance and a reliable supply of buffer fluid. It has already been proposed (DE-B-1525825) that single action, internally pressurised face seal assemblies having a bellows of a resistant plastics material be employed for sealing aggressive media, in which means for applying bias forces are arranged externally so that they will not be exposed to the aggressive medium. Nevertheless, internally pressurised face seal assemblies have the disadvantage that they exhibit a substantially greater degree of leakage than externally pressurised ones thereby making it necessary to take special precautionary measures in regard to an ecologically sound disposal of the leakage when they are used for aggressive media. Furthermore, it is known (EP-A-133073) to provide an elastomeric corrugated bellows unit rotatable with a shaft and having a cutter-like end portion which rests on a stationary sealing surface to provide therewith a sealing relationship by rubbing linearly along said surface when the shaft is rotated. One cannot assume, that the assembly is of particular aptitude for use in aggressive media. Moreover, the attainable sealing effect is restricted to very low pressures of the medium to be sealed and one must expect relatively high levels of leakage. Consequently, there is a need for an externally pressurised face seal assembly, especially for use with aggressive to very aggressive media, without the disadvantages of the known assemblies.
In view of the foregoing it is an object of the present invention to provide an improved sealing element suitable for incorporating in a single action face seal assembly for sealing aggressive media. Another object of the present invention is to provide a sealing element suitable for incorporating in a single action face seal assembly for sealing aggressive media which requires minimum maintenance and having a minimum leakage and very compact dimensions.
For achieving the aforesaid object, the present invention provides a sealing element for a face seal assembly including a bellows portion having mounting portions at the axial ends thereof, one of said mounting portions retains an annular face member, said sealing element on at least the faces thereof exposed to the medium to be sealed being formed of a plastics material selected from a group of plastics material including PEEK (acronym for polyetheretherketone) and ECTFE (acronym for ethylene-chlorotrifluoroethylene copolymers). When a sealing element made in accordance with the present invention is incorporated in a single action face seal assembly, not only can this sealing element fulfill the function of dynamically sealing the face seal assembly, but it can also create the necessary static and dynamic biasing forces that should be applied to the annular face member during operation or when stopped. Since the sealing element in accordance with the invention can fulfill a plurality of functions for which separate members have had to be provided hitherto, the invention enables the construction of a single action, externally pressurised face seal assembly which not only exhibits a substantially reduced level of leakage in comparison with conventional single action face seal assemblies, but it can also have compact dimensions thus making it suitable for installation in equipment of limited spatial capacity. The face seal assembly can readily be designed such as to take into account the requirements of e.g. German Standards DIN 24960. The invention thus is a substantial contribution to the development of economical, ecologically sound seals for aggressive medium for which consideration was primarily given to the employment of complex, double action face seal assemblies in the past. In accordance with the invention, the sealing element is formed throughout of the specific aforesaid plastics material. In addition reinforcing inserts in the form of fibre reinforcements may be embedded in the sealing element. The sealing element in accordance with the invention may take the form of a rotating part for common rotation with a rotating component, e.g. a shaft, or of a non-rotating part to be mounted on a stationary component, e.g. a casing.